Camera tracking; movie clip does not show up as background footage
After tracking and solving press Setup tracking scene, and then "set as backgroud".

That will set the movieclip to be used as image background for camera view.

If you want to set it manually , then just enable images as background, select your video clip as source and go to camera view on the vieport (numpad zero)

Also, before you ask, the background image is used for reference only, to render the image in the backcground read> Cycles not rendering image or movie clip used as background in the viewport